Cytokine pathways mediate a broad range of biological functions, including many aspects of inflammation and immunity. Janus kinases (JAK), including JAK1, JAK2, JAK3 and TYK2 are cytoplasmic protein kinases that associate with type I and type II cytokine receptors and regulate cytokine signal transduction. Cytokine engagement with cognate receptors triggers activation of receptor associated JAKs and this leads to JAK-mediated tyrosine phosphorylation of signal transducer and activator of transcription (STAT) proteins and ultimately transcriptional activation of specific gene sets (Schindler et al., 2007, J Biol. Chem. 282: 20059-63). JAK1, JAK2 and TYK2 exhibit broad patterns of gene expression, while JAK3 expression is limited to leukocytes. Cytokine receptors are typically functional as heterodimers, and as a result, more than one type of JAK kinase is usually associated with cytokine receptor complexes. The specific JAKs associated with different cytokine receptor complexes have been determined in many cases through genetic studies and corroborated by other experimental evidence.
JAK1 was initially identified in a screen for novel kinases (Wilks A. F., 1989, Proc. Natl. Acad. Sci. U.S.A. 86:1603-1607). Genetic and biochemical studies have shown that JAK1 is functionally and physically associated with the type I interferon (e.g., IFNalpha), type II interferon (e.g., IFNgamma), IL-2 and IL-6 cytokine receptor complexes (Kisseleva et al., 2002, gene 285:1-24; Levy et al., 2005, Nat. Rev. Mol. Cell. Biol. 3:651-662; O'Shea et al., 2002, Cell, 109 (suppl.): S121-S131). JAK1 knockout mice die perinatally due to defects in LIF receptor signaling (Kisseleva et al., 2002, gene 285:1-24; O'Shea et al., 2002, Cell, 109 (suppl.): S121-S131). Characterization of tissues derived from JAK1 knockout mice demonstrated critical roles for this kinase in the IFN, IL-10, IL-2/IL-4, and IL-6 pathways. A humanized monoclonal antibody targeting the IL-6 pathway (Tocilizumab) was recently approved by the European Commission for the treatment of moderate-to-severe rheumatoid arthritis (Scheinecker et al., 2009, Nat. Rev. Drug Discov. 8:273-274).
Myeloproliferative disorders (MPD) originate in hematopoietic stem cells and primarily manifest in elevated counts of mostly normal cells of the myeloid lineage. A primary distinction between Philadelphia-chromosome positive (Ph+) and Philadelphia-chromosome negative (Ph−) can be made. Ph+ MPD results in chronic myelogenous leukemia and is driven by a bcr-abl fusion protein that drives hematopoietic cell proliferation. Ph− MPD can be further subclassified into three distinct disorders by related varieties, namely polycythemia vera (PV), essential thrombocythemia (ET) and idiopathic myelofibrosis (IMF). Dameshek, W., Blood 6(4):372-375 (1951). Patients with PV suffer from high counts of red blood cells, whereas patients with ET have high levels of circulating platelets. If left untreated, both diseases can result in life-threatening thrombotic events. Patients with IMF experience fibrosis of the bone marrow with subsequent displacement of hematopoiesis into the spleen and liver. This primarily leads to splenomegaly, which is followed by anemia in later stages of the disease as hematopoiesis becomes non-productive. These patients have a poor prognosis, although under certain conditions they can be cured by means of an allogeneic bone marrow transplant. There is no known cure for Ph− MPD diseases.
An activating mutation in the tyrosine kinase JAK2 is associated with PV, ET, IMF and other diseases. Virtually all patients with PV and about 50% patients with ET and IMF harbor this mutation. Morgan, K. J. and Gilliland, D. G., Ann. Rev. Med. 59:213-222 (2008). The mutation is an exchange from valine to phenylalanine at position 617 in the mature human JAK2 protein (V617F). Additional mutations in JAK2, commonly found in exon 12 and referred to as exon 12 mutations, also have an activating effect and can lead to MPD. Furthermore, a T875N mutation was associated with megakaryoblastic leukemia. Finally, JAK2 fusion proteins have been identified in acute leukemias.
The V617F mutation functions to activate JAK2, which leads to MPD. In non-mutated form, JAK2 is linked to cytokine receptors (i.e. EPO-R, TPO-R and others) and only gets activated if the receptor itself is activated by stimulation with the cognate cytokine ligand. Hematopoiesis as a whole is then regulated through the availability of ligands. For example, the cytokine erythropoietin (EPO) stimulates hematopoietic progenitor cells to give rise to red blood cells. A mutation that uncouples JAK2 activation from EPO, therefore, leads to elevated levels of red blood cells. By analogy, thrombopoietin (TPO) regulates platelet growth by binding to the TPO-R, which in turn also signals through JAK2. Thus, elevated levels of platelets can also result from aberrant JAK2 activation.
Compounds are needed that inhibit JAK2, which would be beneficial to patients with JAK2 driven myeloproliferative disorders, as well as, other diseases that are responsive to the inhibition of JAK2. Such diseases include both diseases in which JAK2 is activated by mutation or amplification, as well as, diseases in which JAK2 activation is a part of the oncogenic cascade. Numerous tumor cell lines and tumor samples have high levels of phospho-STAT3, which is a JAK2 target gene.
JAK3 associates exclusively with the gamma common cytokine receptor chain, which is present in the IL-2, IL-4, IL-7, IL-9, IL-15 and IL-21 cytokine receptor complexes. JAK3 is critical for lymphoid cell development and proliferation and mutations in JAK3 result in severe combined immunodeficiency (SCID) (O'Shea et al., 2002, Cell, 109 (suppl.): S121-S131). Based on its role in regulating lymphocytes, JAK3 and JAK3-mediated pathways have been targeted for immunosuppressive indications (e.g., transplantation rejection and rheumatoid arthritis) (Baslund et al., 2005, Arthritis &Rheumatism 52:2686-2692; Changelian et al., 2003, Science 302: 875-878).
TYK2 associates with the type I interferon (e.g., IFNalpha), IL-6, IL-10, IL-12 and IL-23 cytokine receptor complexes (Kisseleva et al., 2002, Gene 285:1-24; Watford, W. T. & O'Shea, J. J., 2006, Immunity 25:695-697). Consistent with this, primary cells derived from a TYK2 deficient human are defective in type I interferon, IL-6, IL-10, IL-12 and IL-23 signaling. A fully human monoclonal antibody targeting the shared p40 subunit of the IL-12 and 11-23 cytokines (Ustekinumab) was recently approved by the European Commission for the treatment of moderate-to-severe plaque psoriasis (Krueger et al., 2007, N. Engl. J. Med. 356:580-92; Reich et al., 2009, Nat. Rev. Drug Discov. 8:355-356). In addition, an antibody targeting the IL-12 and IL-23 pathways underwent clinical trials for treating Crohn's Disease (Mannon et al., 2004, N. Engl. J. Med. 351:2069-79).